Cause of Yerba Buena Island fire still unclear

San Francisco firefighters are still investigating what sparked Sunday night's dramatic fire on Yerba Buena Island.

The fire, reported at about 7:50 p.m., charred 3 to 4 acres of vegetation but did not burn any structures or cause any injuries, fire department spokeswoman Mindy Talmadge said this morning.

Two eastbound lanes on the lower deck of the Bay Bridge were shut down as crews battled the blaze. Flames were visible to eastbound motorists, and those traveling west could see an orange glow around the island and a plume of smoke.

The fire began just as hordes of San Francisco 49ers fans were headed back to the East Bay after watching Sunday's game at Candlestick Park against the Houston Texans.

Traffic was at a standstill for miles on northbound U.S. Highway 101 as cars -- many decorated with 49ers flags -- inched toward the bridge.

Lanes on the bridge gradually reopened as crews gained the upper hand on the stubborn blaze, Talmadge said.

The fire appears to have started in the bottom of a ravine in a hard-to-reach area, she said.

"The chances are investigators will not even be able to access that area to make an absolute determination" of the cause, Talmadge said.

"It was difficult to access it to suppress it as well," she added, noting that crews also used a fireboat to fight the blaze from the water.

Talmadge said she has heard speculation that fireworks may have sparked the blaze, but that investigators have not found evidence of that.